This invention relates to optical pattern tracing controls and in particular, to controls of the type which repetitively scan the pattern to be followed in a circular manner, derive a signal from said scanning operation and utilize the signal to control coordinate drive motors, which position a machine tool.
There are various types of optical pattern tracing machine control systems including scanning and non-scanning tracing heads, friction and coordinate drive machines, edge and line tracers. This invention has particular application to a pattern tracer of the circular scanning type for operation with a coordinate drive system which may be used for line tracing. By a circular scanning tracer, it is meant a tracer which views the pattern in such a manner that the point observed by the tracing head is caused to rotate repetitively so as to describe a circular path on the surface bearing the pattern when the head is stationary. Naturally, when the head is in translational motion, the point scanned by the tracing head will more closely approach an epicycle. The detector in the tracing head is arranged to produce a signal indicative of a change of illumination of the detector. The signal representing this transition is then processed and used to control a pair of motors, which when associated with the suitable machinery, will cause the tracing head and related machine tool to move in a plane in accordance with the pattern as controlled by the motors.
It will be appreciated that when, in accordance with general practise in this field, the convolutions performed by the tracing head are similarly performed by the machine tool, which may, for example, be a cutting torch, the material to be cut will be shaped into the same form as the pattern being traced by the tracing head.
In circular scanning tracing heads, it is necessary to correlate the signal produced by the scanning head with information indicating the instantaneous direction of the scanning point. In the prior art, this reference information has been generated as sinusoidal information, as for example, in the tracing head described in Canadian Pat. No. 917,773 issued Dec. 26, 1972 in the names of Robert E. Parker, et al. In order to permit the machine to continue operating in a particular direction in the absence of a line to prevent inadvertent reversal in the presence of unintended breaks in the line or in the presence of intentional breaks which are required for other purposes, it has been a practise to incorporate a memory circuit such as that described in Canadian Pat. No. 950,553 issued July 2, 1974 in the names of William Dell, et al, or in Canadian Pat. No. 1,005,549 issued Feb. 15, 1977 in the names of Francis P. Murphy, et al. It will be assumed that the system operates in accordance with these latter systems as described in the aforementioned patents and that the system has been arranged to respond to command marks as described in Canadian Pat. No. 1,005,549 and in accordance with Canadian Pat. No. 950,553.
In accordance with the previous patents, when the system encountered a command mark, it would step the program on to the next step causing certain controlled functions to occur. However, the program was stored in a matrix type of memory which required a separate step for each function irrespective of whether the functions were repetitive. In situations where a number of corners are encountered, it is necessary in some circumstances to reduce the speed of the system when negotiating each such a corner. The essentially repetitive nature of this function indicates it would be preferable not to store it as a repeated function in the program matrix.